A quantum cascade laser is a unipolar device. It utilizes intersubband transitions, unlike the traditional direct band gap semiconductor lasers, and it usually emits in the mid-infrared (“mid-IR”) or far-infrared (“far-IR”) wavelength range.
Mid-IR sources are of interest for various reasons. Strong absorption lines in the mid-IR region from the vibration of chemical bonds can be used to identify molecular composition. For example, FIG. 1 (prior art) shows a strong absorption line of CO2 near 4.3 um. A single wavelength Mid-IR light source such as a QCL can be used to detect gas molecules such as CO2 by detecting the absorption of a characteristic wavelength such as 4.3 um.
To achieve single wavelength emission, grating structures may be added to the QCL in the active region to make a distributed-feedback (“DFB”) quantum-cascade laser (“DFB QCL”). DFB QCLs generally emit a single wavelength and can only be tuned over a small wavelength range, which allows them to be used to detect a single species of small gas molecule such as CO2. However, some big molecules in solid or liquid phases have wide and/or complex absorption spectra, like the explosive substances in FIG. 2, for example, which shows infrared absorption spectra for PETN 102, RDX 104, TATP 106 and TNT 108. For detecting and differentiating substances with such wide and/or complex absorption spectra, QCLs with both single wavelength emission and wide tuning range are desirable. Range R indicated in the figure, for example, may be used to detect, and differentiate among, the spectra shown.
External cavity QCLs can have both single wavelength emission and wide tuning range, but they are usually expensive and bulky. A distributed Bragg reflector (“DBR”) QCL has one or both reflective gratings outside the gain region of the laser, allowing for a more independent thermal tuning of the gratings and a wider tuning range than a DFB QCL. A DBR QCL is thus a potential alternative to external cavity QCLs with the advantages of relatively low cost and a compact, robust, monolithic form.
DBR QCLs typically have an essentially uniform, common core, as shown in FIG. 3 (prior art). The grating layers on DBR sections are formed directly on the layer(s) of the common core. Since the region of common core under the DBR is passive in operation (not part of the gain region) receiving no or minimal pump current during operation (due to additional associated current blocking structures or the like), it has relatively strong resonant absorption.
Implementing a waveguide different from the waveguide of the active region in a DBR QCLs is disclosed in the related applications referenced above. By using for the DBRs a different waveguide transparent (or at least more transparent than the active region waveguide) to wavelengths in the operating wavelength range, absorption losses in the DBRs can be reduced, allowing higher maximum power and wider total tuning (lasing) range in the laser device.